The present invention relates to a hydraulic shock absorber.
Conventionally, there is proposed a so-called self-leveling hydraulic shock absorber, i.e., a hydraulic shock absorber having a self-leveling function for automatically maintaining a constant standard height of a vehicle, an example of which is a hydraulic shock absorber disclosed in Japanese Patent Application Public Disclosure Hei 10-138730.
The self-leveling hydraulic shock absorber disclosed in Japanese Patent Application Public Disclosure Hei 10-138730 comprises a pumping means for supplying a hydraulic fluid in an oil tank into a cylinder by utilizing extension and contraction of a piston rod, and a returning means for returning the pressure hydraulic fluid from the pumping means and the cylinder into the oil tank according to a position of the extending and contracting piston rod. Oscillation of a suspension apparatus while the vehicle is running actuates the pumping means and the returning means, whereby a constant standard height of the vehicle is automatically maintained by appropriately adjusting a pressure in the cylinder to maintain a constant extension length of the piston rod.
In such a self-leveling hydraulic shock absorber, an inside of an oil tank or a reservoir is divided into a plurality of chambers by a diaphragm (flexible membrane) or a free piston. A gas is sealingly contained in one of the chambers, and a hydraulic fluid is sealingly contained in the other of the rooms, so that the gas is prevented from mixing with the hydraulic fluid.
In some conventional hydraulic shock absorbers, an oil tank or a reservoir is divided into a plurality of gas chambers by a diaphragm or a free piston. A first gas having a first pressure is sealingly contained in one of the gas chambers, and a second gas having a second pressure which is different from the first pressure is sealingly contained in the other of the gas chambers. In such a conventional hydraulic shock absorber, a gas-supplying hole is formed through an outer wall of a cylinder to supply a gas having a predetermined pressure into the gas chamber in the oil tank or the reservoir. In making the hydraulic shock absorber, after the hydraulic shock absorber is assembled by placement of components, the gas having the predetermined pressure is supplied from the gas-supplying hole into the gas chamber, and then a plug is welded in the gas-supplying hole to sealingly close the gas chamber. However in this method, undesirable effects may be caused as a result of heat and sputter generated by welding, such as deterioration of the rubber diaphragm or malfunction of the sliding surface of the free piston. In addition, contact of the tip of the plug might damage the diaphragm.